1. Field of Invention
The present invention relates to a device for detecting a misfiring state of an engine, and a method of detecting the misfire.
2. Description of Related Art
An engine may suffer from a “misfire”, which is an incomplete combustion of an air-fuel mixture in a combustion chamber that takes place under certain conditions such as an operating temperature of the engine and a state of a spark plug. When a misfire takes place in the combustion chamber, not only the operating speed of the engine drops, but also incomplete combustion gases are exhausted from the combustion chamber into the atmosphere. Frequent occurrence of the misfire disables the engine to produce its nominal torque. Further, it is desired to reduce the amount of the incomplete combustion gases exhausted from the combustion chamber, from the standpoint of preventing air pollution.
In view of the above, the operation of an engine is controlled by an electronic control unit (ECU), by controlling the ratio of an air-fuel mixture and the compression ratio within the combustion chamber, so as to prevent a misfire in the combustion chamber. In addition, the ECU monitors the operating state of the engine, to detect an occurrence of the misfire, and activates a suitable alarm indicator when the frequency of occurrence of the misfire has exceeded a threshold, so that the user of the engine is informed of an abnormal combustion state of the engine and is prompted to take an immediate inspection or repair of the engine as needed.
In a multi-cylinder engine, for example, the above-indicated monitoring of the misfire is implemented as described below. Namely, an occurrence of a misfire in a given cylinder causes a drop of the operating speed of the engine in the combustion cycle involving the misfiring cylinder. As a result, a time required for the engine output shaft or crankshaft to rotate a predetermined angle (° CA) is increased in the combustion cycle involving the misfiring cylinder. This increased time is compared with a time required for the crankshaft to rotate the predetermined angle in the last combustion cycle involving another cylinder. If a difference between these two times is larger than a threshold value, it is determined that a misfire has occurred in the present combustion cycle. If the frequency of occurrence of the misfire has exceeded the threshold, it is detected that the engine is suffering from the misfire.
However, a variation of the operating speed of the engine does not take place solely due to a misfire in the combustion chambers of the engine. Where the engine is installed on an automotive vehicle, for instance, the operating speed of the engine may vary upon application of an abrupt brake to the vehicle or during running of the vehicle on a rough road. It may occur that a detection of a misfire of the combustion engine is not normally performed, due to the variation of the operating of the operating speed of the engine, caused by the above-described abrupt brake or running on a rough road. The electronic control device may be arranged such that once the alarm indicator such as an alarm lamp has been activated as a result of detection of the misfiring state of the engine. The alarm indicator cannot be turned off, unless the engine has been inspected or repaired by the manufacturer of the engine or a service shop.
Various misfire detecting devices have been proposed to avoid an erroneous detection of a misfire in the engine. JP-A-6-307284 discloses an example of such misfire detecting devices. The misfire detecting device in this example is adapted to measure a time required for the crankshaft to rotate a predetermined angle in the combustion cycle of each cylinder of the engine, and determine that there is a possibility of the misfire in the present combustion cycle if a difference of the time measured in the present combustion cycle with respect to that in the last combustion cycle is larger than a predetermined threshold. This misfire detecting device is further adapted to obtain a pattern of variation of the time in each of the successive combustion cycles during the past period back to the crank angle position of 720° CA. On the basis of the thus obtained pattern of variation of the time, the conventional misfire detecting device determines whether a misfire has occurred in the present combustion cycle. This arrangement permits accurate monitoring of the misfiring state of a multi-cylinder engine installed on an automotive vehicle, for example, without a risk of erroneous detection of the misfiring engine even when an abrupt brake is applied to the running vehicle.
To monitor the operation of the engine for detecting a misfiring state of the engine immediately after the starting of the engine, it is required to accurately detect a moment of transition of the operating state of the engine to its self-sustaining state (firing state). However, it is difficult to accurately detect the moment of transition of the engine to its firing state. Therefore, the electronic control device (ECU) is arranged to determine that the engine has entered into the firing state when the operating speed of the engine has reached a predetermined value. In this arrangement, a misfire monitoring routine is initiated by the ECU upon determination that the engine has entered into the firing state, so that the subsequent operation of the engine is monitored for occurrence of a misfire.
On the other hand, the moment at which the engine has actually entered into the firing state may differ from the moment of determination by the ECU that the engine has entered into the firing state after starting of the engine. This difference may cause the following problems. That is, after the ECU has initiated the misfire monitoring routine as a result of its determination that the engine has entered into the firing state after the starting of the engine, the engine may actually be turned off without establishment of its firing state. In this case, misfire-frequency data indicative of the frequency of occurrence of a misfire of the engine during a period between the above-indicated two moments are stored in a memory incorporated in the ECU, and are kept in the memory as long as the ECU is held on, that is, until electric power is removed from the ECU. If the engine starting that does not result in the firing state of the engine is repeated, the frequency of occurrence of the misfire as represented by the stored misfire-frequency data is increased, leading to a risk that the ECU erroneously determines that the engine is in the misfiring state, while in fact the engine has not entered into the firing state.